Apparatus for heating the surface of constructions

ABSTRACT

An economical apparatus is provided for heating the surface of constructions such as a road, floor, wall, etc. which comprises at least two juxtaposed ferromagnetic pipes having a wall thickness of at least twice the skin depth of a.c. flowing therethrough and an insulated electric wire passing in series through each inside of said ferromagnetic pipes, both the ends of which are connected to an alternating current source. When an alternating current is passed through said insulated electric wire, a secondary current is induced along both the inner and outer skin portions of said ferromagnetic pipes and heat is generated therein. Further, the electrically neutral points of the ferromagnetic pipes are connected to each other by an electric conductor and a fault detecting means responsive to a flow of current in the conductor is provided to disconnect the insulated wire from the alternating current source upon occurrence of an electrical fault in the apparatus.

United States Patent [191 IdO Dec. 18, 1973 APPARATUS FOR HEATING THESURFACE Primary Examiner-A. Bartis OF CONSTRUCTIONS Attorney-Fred C.Philpitt [75] inventor: Masao Ando, Yokohamashi, Japan 73 A Chi C 1 O k.1 [57] ABSTRACT 1 sslgnee' m sa apan An economical apparatus isprovided for heating the [22] Filed: Nov. 2, 1971 surface ofconstructions such as a road, floor, wall,

etc. which comprises at least two juxtaposed ferro- [211 Appl' 194366magnetic pipes having a wall thickness of at least twice the skin depthof a.c. flowing therethrough and [52] US. Cl 219/l0.49, 219/300,219/519, an insulated electric wire passing in series through 317/18 Deach inside of said ferromagnetic pipes, both the ends [51] lnt.Cl 1105b9/00, HOS b l/ 02, H02h l/02 of which are connected to an alternatingcurrent [58] Field of Search 219/ 10.49, 10.51, source. When analternating current is passed through 219/300, 301, 514, 519; 317/18 A,18 D said insulated electric wire, a secondary current is induced alongboth the inner and outer skin portions of [56] References Cited saidferromagnetic pipes and heat isgenerated therein. UNITED STATES PATENTSFurther, the electrically neutral points of the 3,515,837 6/1970 Ando219/l0.49 erromagnetic pipes are connected to each other by an 3,622,731ll/197l Ando 219/1049 electric conductor and a fault detecting means12/1968 Beckius 219/1049 X responsive to a flow of current in theconductor is AI'IdO-i provided to disconnect the insulated wire from the#F' alternating current source upon occurrence of an lce r 3,410,97711/1968 Ando. 219/10.49 elecmcal'fault m the apparatus 1 Claim, 5Drawing Figures 9 1 A.C. U CE ,5) I so g PIPES 39 ELECTRICALLYNGINSULATI 38 FAULT DETECTOR PATENTEUBEE 18 ms 3,780,250

sum 1 or 2 FIG. I PRIOR ART APPARATUS FOR HEATING THE SURFACE OFCONSTRUCTIONS The present invention relates to an improvement in theheat-generating pipes utilizing skin effect current, disclosed in U.S.Pat. No. 3,410,977 and U.S. Pat. No. 3,515,837. The inventor andassignee of these patents are the same with those of the'presentinvention.

Firstly the background of the present invention will be given referringto the accompanying drawings.

FIG. 1 shows the principle of the heat-generating pipe utilizing skineffect current disclosed in the abovementioned U.S. Pat. No. 3,410,977.In this figure, numeral l shows a ferromagnetic pipe such as a steelpipe, and terminals 6 and 6' at both the ends of the pipe are connectedto an alternating current source 5 by an electric wire 2 and aninsulated electric wire 2 which passes through the inside of .the pipe.

When the resistivity of the material of the ferromagnetic pipe 1 is pcm); the specific permeability thereof is [1,; and the frequency of thealternating current source is f (Hz), the depth of skin of alternatingcurrent (a.c.), S (cm), is expressed by the following formula:

If there is a following relationship (2) among the wall thickness t(cm), the inner diameter d (cm) and the length l (cm) of the pipe 1 andthe above-mentioned S:

the current 3 supplied from the a.c. source 5 flows concentratedlythrough the inner skin portion of the ferromagnetic pipe I, and heat isgenerated only at the inner skin portion. In this case, substantially novoltage appears on the outer surface of the pipe, and hence even if theouter surface is contacted with human beings or animals, they sustain noinjury, and also, the heat-generating pipe does not become a cause offire. Thus, the heat-generating pipe can be utilized as a safeheat-generating pipe for heating e.g. the surface of a road, floor,wall, etc.

FIG. 2 shows the heat-generating pipe utilizing skin effect currentdisclosed in the above-mentioned U.S. Pat. No. 3,515,837. In thisfigure, numerals 7 and 8 show ferromagnetic pipes having the samespecified conditions relative to the wall thickness and length as thoseof the pipe 1 of FIG. 1, the adjacent ends of the two pipes being eachconnected by a connecting means 13 or 14 having as low as impedance aspossible. Numeral 9 shows an insulated electric wire passing through theinsides of the ,two ferromagnetic pipes and connected to the terminalsof the a.c. source'12. In this apparatus, thealtemating current 10passing through the electric wire 9 is a primary current, and thesecondary current flows through the inner skin portions 11 of theferromagnetic pipes to generate heat therein, and substantially novoltage appears on the outer surfaces of the ferromagnetic pipes 7 and8, as in the apparatus shown in FIG. 1. Thus, this apparatus can be alsoutilized as a safe heat-generating pipe. Although the above-mentionedexplanation is made referring to single phase circuits, the principle ofthe polyphase circuit also is same as those of the simple phasecircuits.

The advantages of the apparatuses shown in FIG. 1 and FIG. 2 over the MIcable or a plastic materialinsulated cable which are usually used bydirectly laid under the surface of road, floor or the like, are that theinsulated electric wires, which are liable to suffer damages, areprotected by ferromagnetic pipes having a considerable mechanicalstrength; that, even if the in sulated electric wire passing through theinside of the ferromagnetic pipe should suffer damages, they are readilyreplaced by new wires without digging up the road, floor, wall or thelike; and that insulated electric wires of lower heat resistance orthinner electric wires are sufficient for the apparatuses shown in FIG.1 and 2, because the major part of heat is generated at the inner skinportion of the ferromagnetic pipe, and the ferromagnetic pipe iscontacted directly with a substance to be heated to make the transfer ofheat easy, and hence the temperature of the insulated electric wireinside the pipe is relatively low.

However, the known heat-generating apparatuses shown in FIG. I and 2have a drawback in that heat is generated only at the inner skin portionof the ferromagnetic pipe, and the outer skin portion is not utilized.Further, in the apparatus of FIG. I, the larger the surface area to beheated, beneath which the ferromagnetic pipe I is distributed, thegreater the number of bend parts to be provided. Thus, it becomesnecessary to connect straight pipes with bend pipes by expensiveon-the-spot welding. Also in the apparatus of FIG. 2, the connection bywelding of the electric wires 13 and 14 having a low impedance is, as amatter of fact, carried out on the spot, and the cost becomes higherthan the material cost of the ferromagnetic pipe such as steel pipe, insome cases.

The object of the present invention is to provide an apparatus forheating the surfaces of constructions such as a road, floor, wall, etc.which does not require expensive on-the-spot working, is safe andutilizes both the inner and outer skinportions of the ferromagnetic pipeas heat-generating portions, whereby about twice the quantity of heatgenerated by the above-mentioned known heat-generating apparatuses canbe obtained.

The present invention resorts to an apparatus for heating the surface ofconstructions which comprises at least two juxtaposed ferromagneticpipes having the wall thickness of at least twice the skin depth of a.c.flowing therethrough, and an insulated electric wire passing in seriesthrough each inside of said ferromagnetic pipes, both the ends of whichare connected to an alternating current source, whereby when analtemating current is passed through said insulated electric wire, asecondary current is induced at both the inner and outer skin portionsof said ferromagnetic pipes to generate heat there.

As the insulated electric wire passing through the inside of the pipes,there can be illustrated copper or aluminum wires coated with aninsulated material such as organic high molecular weight substances.

As the surface of constructions, there can be illustrated the surfacesof a road, floor, wall, etc.

FIGS. 1 and 2 are schematic cross-sectional views for illustrating theprinciples of known heat-generating pipes utilizing skin effect current.

FIG. 3 is a schematic view for illustrating the principle of theinduction heat-generating pipe of the present invention. 1

FIG. 4 is a schematic plan view of the apparatus of the presentinvention. v

FIG. 5 is a cross-sectional view of FIG. 4 along the VV line.

Next, the present invention will be further illustrated referring to theaccompanying drawings.

The schematic view shown in FIG. 3, illustrating the principle of thepresent invention is the same as that shown in FIG. 2 except that thelow impedance connections 13 and 14 of FIG. 2 are omitted. Thus, in theapparatus of FIG. 3, voltage appears alsoon the outer surfaces 19', 20'of ferromagnetic pipes 15 and 16. FIG. 3 will be explained further indetail. An insulated electric wire 17 passing through the inside of theferromagnetic pipe 15,.16, forms a primary circuit together with an a.c.source 21, and an electric current 18 flows therethrough. Secondaryelectric current is induced in each of the ferromagnetic pipes 15, 16,and a circuit of the current is formed between the inner skin portion 19and the outer skin portion 19' in the ferromagnetic pipe 15, whileanother circuit of the current is formed between the inner skin portion20 and the outer one 20' in the ferromagneticpipe 16, to generate heatat both'the innerskin portion and the outer one. Such a secondarycurrent is nothing but an eddy current.

In order to make the best use of the present inven tion, it is necessarythat the wall thicknesses of the above-mentioned ferromagnetic pipes 15,16 are at least twice the skin depth of a.c. flowing therethroughexpressed by the formula (1). I

As mentioned above, voltage appears also on the outer surface of theferromagnetic pipe, in the apparatus of FIG. 3 on which the presentinvention is based, and hence if the substance to be heated is of a lowresistivity, as in' metal, there is a danger that the electric currentflows'to the substance to be heated.

However, in the heating of the surface of a road,

floor, wall or the like, the substance surrounding the stance having asimilar property, and hence if the voltage appearing on the outersurface of the ferromagallowable in the above-mentioned example, can beextended up to about I00 m.

FIG. 4 shows an embodiment of the present invention. In this figure,numeral 25 shows ferromagnetic pipes corresponding to l5, 16 in FIG. 3,and they are arranged in six parallel rows, the number of which,however, can be varied dependent on the area to be heated. The adjacentends of these ferromagnetic pipes are connected by bends 32, 33, 34 madeof an insulating material and a connecting box 35 so that theferromagnetic pipes, the bends and the box can form a serial pipe. Bends32, 33, 34 may be made of an electrically conductive material, such assteel, providing that insulating pieces are inserted in between theferromagnetic pipes and the bends. Numeral 26 shows an insulatedelectric wire passing through the inside of the abovementioned serialpipe and connected to electric wires 27, 28 which are connected to ana.c. source 36, at terminal boxes 30, 31 provided at both the ends ofthe se.--

rial pipe.

The bends 32, 33, 34 and connecting box 35, when the heating apparatusis used for preventing the surface of a road from freezing, for meltingsnow thereon or for the like purposes, are provided on the sides of roador the shoulder of road which is the part where there is not so frequentwalking of human being or animals or traffic.

In the apparatus shown in FIG. 4, the left and/or right side ends offerromagnetic pipes 25 may be accommodated in a duct or ducts allowingthe insulated conductor 26 to pass therethrough. In this case, the endsof the ferromagnetic pipes must be electrically isolated from the ductor ducts.

In FIG. 5 showing a. cross-sectional view along the VV line of FIG. 4,the ferromagnetic pipes 25 are surrounded by an'insulating substance ora substance having a similar property 43 which constitutes road, floor,wall or the like, such as asphalt, concrete, etc.

The ferromagnetic pipes may be applied by an insulation coating 44.

As mentioned above, the heating apparatus of the present invention is ofa construction in which voltage appears also on the outer surface of theferromagnetic netic pipe is not so high, concrete, asphalt, etc. func- Ition as an insulating substances which do not give danger to human beingor animals. Thus, the apparatus of FIG. 3 can be utilized as byheat-generating apparatus as well as those shown in FIGS. 1 and 2.

The length of each ferromagnetic pipe as required in the heating of thesurface of a road, floor, wall or the like, is fortunately about l m toabout-2O m. When steel pipes having a length of this extent are used asthe ferromagnetic pipe and an alternating current of a commercialfrequency of -60 Hz is used as an a.c. source, the voltage appearing onthe outer surface of each ferromagnetic pipe is 0.5V to 5V, and hencethe apparatus can be fully utilized for the above-mentioned purposes.

In the apparatus of FIG. 3, an insulation coating can be further appliedon the outer surface of the ferromagnetic pipe. As the insulationcoating, there can be illustrated coating with paint, coaltar, or'thelike, lining with polyvinyl chloride, polyethylene, or the like, orwinding of insulating tape around the pipe. By applying such'aninsulation coating, thelength of the steel pipe pipe, and hence if afault should occur on the insulated electric wire, there is apossibility that the outer surface of the ferromagnetic pipe has apotential close to the voltage of the a.c. source dependent on the placeof the fault. 1 i i V Since the voltageof the a.c. source is far greaterthan that appearing on the outer surfaces of the respective pipes, suchfault could lead to more serious accidents in the vicinity of the faultpoint unless it is detected so that any countermeasure can be taken. Forthis reason, some device capable of detecting faults should beincorporated into the heating pipe arrangement in the presentapplication. For the purpose of providing such a device, a voltagedetector is connected in the pipe arrangement as shown in FIG. 4.

As a method for preventing troubles from occurring from theabove-mentioned accident, each one point, preferably each oneelectrically neutral point, on the outer surface of each ferromagneticpipe is connected to each other by an electrical conductor which isfurther connected to a voltage-detector. One example thereof isillustrated in FIG. 4, where each ferromagnetic'pipe is connected toeach other byan electric wire 39, at each one electrically neutral point42 on the outer surface of each pipe, that is, each one point on theouter surface of the middle part in the longitudinal direction, of eachsteel pipe, and the electric wire 39 is further connected by an electricwire 40 to one terminal of a voltage-detector 38, another terminal ofthe detector being connected by an electric wire 41 to a specific pointof voltage at an ac. source 36. When the heating apparatus of thepresent invention is under normal operation, no voltage occurs in theelectric wire 39 or no current flows therein, and hence the detector 38for detecting a fault does not operate.

However, if a fault should occur in the insulated electric wire 26 inone of the ferromagnetic pipes 25, the potential of the ferromagneticpipe is elevated to generate an electric current in the electricalconductor 39 connected to the pipe, followed by actuating the detector38, and breaking the connection with the ac. source by means of abreaker 37.

Although such a fault in the electric wire 26 can be detected by a knownfault-detector and circuit, the detection by way of such a conventionaldetecting circuit becomes difficult or expensive if the ferromagneticpipes are surrounded by a substantially insulating material such asconcrete or asphalt. However, according to the above-mentioned method ofthe present invention, an easy and highly sensitive detection ispossible.

As mentioned above, it is preferable that each one point on the outersurface of each ferromagnetic pipe 25, to be connected by the connectingconductor 39, is at the neutral point of the pipe or near this point.The reason will be explained asfollows referring to FIG. 3.

When the connection 22 carried out at each neutral point of theferromagnetic pipes 15, 16 or near this point, is compared withtheconnection 22 carried out at each one adjacent end of the pipe, thepotential difference between the other adjacent ends 23 and 24 in theformer case becomes half that of the latter case.

What is claimed is:

1. An apparatus for heating the surface of constructions of anelectrically insulating nature which comprises, at least two juxtaposedferromagnetic pipes each having a length ranging from about one meter toabout twenty meters and wall thickness of at least twice the skin depthof the alternating current flowing therethrough, an insulated electricconductor passing in series through the inside of each of saidferromagnetic pipes, the ends of said conductor being connected to theopposite terminals of an alternating current source, said pipes beingelectrically separated from each other, the respectiveelectrically-neutral points .of the outer surfaces of said ferromagneticpipes being connected to each other by a further electric conductor, andvoltage detecting means responsive to'a flow of current in said furtherconductor to detect a fault in said insulated electric conductor anddisconnect said insulated conductor from said alternating currentsource, whereby when an alternating current from the source is passedthrough said insulated electric conductor a secondary current is inducedin both the inner and outer skin portions of said ferromagnetic pipes togenerate heat there.

1. An apparatus for heating the surface of constructions of anelectrically insulating nature which comprises, at least two juxtaposedferromagnetic pipes each having a length ranging from about one meter toabout twenty meters and wall thickness of at least twice the skin depthof the alternating current flowing therethrough, an insulated electricconductor passing in series through the inside of each of saidferromagnetic pipes, the ends of said conductor being connected to theopposite terminals of an alternating current source, said pipes beingelectrically separated from each other, the respectiveelectrically-neutral points of the outer surfaces of said ferromagneticpipes being connected to each other by a further electric conductor, andvoltage detecting means responsive to a flow of current in said furtherconductor to detect a fault in said insulated electric conductor anddisconnect said insulated conductor from said alternating currentsource, whereby when an alternating current from the source is passedthrough said insulated electric conductor a secondary current is inducedin both the inner and outer skin portions of said ferromagnetic pipes togenerate heat there.